Basal-like breast cancer is the most aggressive subtype of human breast cancer with a higher malignant grade, increased metastatic tendency and poor prognosis. Current treatment options are extremely limited, and new targeted therapy is urgently needed. Recent advances suggest that basal breast cancer originate from the mammary luminal progenitor population, and human basal-like breast tumors and luminal progenitor cells share a similar molecular signature. The objective of this study is to understand the signaling events that regulate mammary luminal progenitor cell fate and how disruption of these regulatory program contributes to basal breast cancer progression, with an emphasis on how the crosstalk between TGF? and STAT5 signaling is coordinated by SnoN. The mammary luminal progenitor cells are derived from the mammary stem cells and are responsible for the massive epithelial expansion during the pregnancy/lactation cycles in the adult glands. Little is known about the molecules/pathways that control the establishment and maintenance of this luminal progenitor fate. Past studies have shown that epithelial expansion during pregnancy is regulated by growth factors and hormones, including TGF? and prolactin. While prolactin, signaling through STAT5, promotes alveologenesis and lactogenesis, TGF? functioning via the Smads, inhibits them. This negative effect of TGF? must be suppressed during late pregnancy to ensure proper alveologenesis. How TGF? signaling is suppressed and coordinated with /STAT5 pathway at this critical juncture has not been defined. SnoN is a critical negative regulator of TGF? signaling by binding to the Smads and repressing their transcription activity. We found that in the mammary gland, SnoN expression is sharply upregulated at late pregnancy and early lactation. Using mouse models with altered expression of SnoN as well as a 3D differentiation model system, we have identified a previously unrecognized role of SnoN in enabling alveologenesis and onset of lactation, likely through promoting Stat5 stability and activation. Since Stat5 is required for the establishment of luminal progenitor cells, we hypothesize that SnoN regulates alveologenesis by promoting luminal progenitor cell fate through co-ordinating the activities of STAT5 and TGF? signaling: by enhancing STAT5 expression and activation and at the same time suppressing Smad signaling. In basal breast cancer cells, aberrant regulation of SnoN and STAT5 expression may promote tumorigenesis. Two specific aims have been designed to test this hypothesis in vivo and in vitro. Aim I will determine the importance of SnoN regulation of STAT5 and Smads in mammary alveologenesis and tumorigenesis in vivo using mouse models. In aim 2, we will determine the molecular mechanism by which SnoN enhances STAT5 expression and activation. Our study may identify important determinant of luminal progenitor cell fate and key pathways that drive basal breast cancer progression. This may facilitate development of novel therapeutic drugs for the treatment of basal breast cancer.